A sudden collapse in the transport lifetime acros. The topological phase transition in (Bi _1-x in _x ) ₂ Se ₃

Topological insulators are newly discovered states of matter with robust metallic surface states protected b. The topological properties of the bulk wavefunctions. A quantum phase transition from a topological insulator to a conventional insulator and a change in topological class can occur only whe. The bulk bandgap closes. In this work, we have used time-domain terahertz spectroscopy to investigat. The low-frequency conductance in (Bi _1-x Ins we tune through this transition by In substitution. Above certain substitution levels we observe a collapse in the transport lifetime that indicate. The destruction of the topological phase. We associate this effect wit. The threshold where states from opposite surfaces hybridize. The substitution level of the threshold is thickness dependent and only asymptotically approache. The bulk limit x≈0.06 where a maximum in the mid-infrared absorption is exhibited. This absorption can be identified wit. The bulk bandgap closing and change in topological class. The correlation length associated wit. The quantum phase transition appears at the evanescent length of the surface states. The observation of the thickness-dependent collapse of the transport lifetime show. The unusual role that finite-size effects play in this topological quantum phase transition.